1. Field of the Invention
The present invention relates to an active type display device employing a thin film transistor (TFT) to drive an emissive element such as an organic electroluminescence (EL) element.
2. Description of Related Art
Organic EL elements are ideal for thin display configurations as they emit light and can eliminate need for a backlight as required in liquid crystal displays, and because they have unlimited viewing angles. Thus, wide application of organic EL elements is expected in the next generation of display devices.
As shown in an example in FIG. 1, an organic EL element 51 can be constituted by forming an anode 53 comprising a transparent electrode made of ITO (Indium Tin Oxide) or the like on a transparent glass substrate 52, and forming, between the anode 53 and a cathode 54 comprising an MgIn alloy, a hole-transport layer 55 comprising MTDATA (4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylphenylamino)triphenylamine), an emissive layer 56 comprising TPD (N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-di(3-methylphenyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine) and Rubrene, and an electron-transport layer 57 comprising Alq3 (8-hydroxyquinoline aluminum), which are stacked in this order. Holes injected from the anode 53 and electrons injected from the cathode 54 are recombined within the emissive layer 56 to emit light, which is radiated outward from the transparent anode side in the direction shown in the figure.
Display devices employing an organic EL element configured as described above can be classified into a passive type having a simple matrix structure and an active type using TFTS. In the active type device, a circuit structure illustrated in FIG. 2 is employed for each pixel.
Referring to FIG. 2, numeral 70 indicates an organic EL element. A driver circuit corresponding to one pixel is constituted by a switching TFT 71 to be switched on and off according to a selection signal SCAN, in which a display signal DATA supplied from a data line 75 is applied to a drain and the selection signal from a selection signal line 76 is applied to a gate, a capacitor 72 connected between a source of the TFT 71 and a predetermined dc voltage Vsc, which is charged with a display signal supplied when the TFT 71 is ON and which holds a charging voltage VG when the TFT 71 is OFF, and a driving TFT 74 in which a drain is connected to a power source line 77 that supplies a drive power source voltage Vdd. A source is further connected to the anode of the organic EL element 70, and a hold voltage VG is supplied to a gate of the TFT 74 from the capacitor 72 to drive the organic EL element 70. In this example, the cathode of the organic EL element 70 is connected to a ground (GND) potential, and the drive power source voltage Vdd is set to a positive potential, 10V for example. Also, the voltage Vsc is of the same potential as Vdd or the ground (GND) potential.
In the above mentioned configuration, a voltage supplied from the data line 75 when the TFT 71 is ON is applied to the gate of the driving TFT 74, and a current in accordance with this voltage flows through the EL element 70 which then emits light.
Video signals can be classified into an analog signal and a digital signal. None of active type organic EL display devices have been heretofore applicable to a digital video signal. On the other hand, a liquid crystal display device applicable to a digital video signal is conventionally configured as shown in FIG. 3.
Specifically, digital video data D0 to D3 supplied from outside are captured in accordance with a shift clock from a shift register 30 into a data register 31, which latches the received data to a latch circuit 32. The digital data thus latched are then converted by a D/A converter 33 into an analog display signal, which is then supplied to a data line 34. To the data line 34, a drain of a TFT 36 which is opened and closed in accordance with a selection signal from a selection signal line 35 is connected. A pixel electrode 370 and a storage capacitor 38 are connected to a source of the TFT 36. A common electrode (371) faces the pixel electrode 370, and a liquid crystal 37 fills between both electrodes.
In the conventional liquid crystal display device to which a digital video signal is input, the driver circuit includes a D/A converter for converting a digital signal into an analog signal, as described above. For instance, when the bit number of digital data is 4, xe2x80x9c16xe2x80x9d stages of gray scale voltage (tone scale voltage) V0 to V15 are necessary for converting the digital data into an analog signal.
Therefore, in order to integrate the driver circuit within a display panel, the display panel must include terminals and wiring patterns for inputting as many as 16 stages of gray scale voltage V0 to V15, thereby failing to downsize the panel. Even if the driver circuit is configured as an IC, the number of terminals for the IC increases.
The same problem is found in an active type EL display device in which each pixel has a circuit structure such as shown in FIG. 2 and the EL element can be driven by a circuit similar to the driver circuit of FIG. 3.
The present invention was made to solve the aforementioned problem of the related art, and aims to provide an active type EL display device which, when a digital video signal is input thereto, does not need the gray scale voltage in accordance with the bit number of the digital data.
In accordance with one aspect of the present invention, there is provided an active matrix type emissive display device including, for each pixel, a thin film transistor to be opened and closed according to a selection signal and an emissive element connected to a data line via said thin film transistor for emitting light according to a supplied current, comprising n transistors for receiving, bit by bit, n bit input digital data sequentially captured for each data line and respectively outputting a different electrical current, wherein a current signal corresponding to a total current amount from said n transistors is output to a corresponding data line and is supplied to said emissive element via said thin film transistor.
In accordance with another aspect of the present invention, there is provided an active matrix type electroluminescence display device comprising an electroluminescence element connected to a data line via a thin film transistor to be opened and closed according to a selection signal, a data register for sequentially capturing input digital data for each data line, a latch circuit for holding the data received by said data register for a predetermined period, and n switches (n: an integer value greater than or equal to 2) to which n bit of digital data for each data line is input bit by bit as a control signal, each of said n switches supplying a different amount of electrical current.
In accordance with another aspect of the present invention, there is provided a driver circuit for an electroluminescence display device including, for each pixel, a thin film transistor to be opened and closed according to a selection signal and an electroluminescence element connected to a data line via said thin film transistor for emitting light according to supplied electrical current, said driver circuit comprising n transistors for capturing, bit by bit, n bit input digital data sequentially supplied for each data line and for respectively outputting a different amount of electrical current, wherein a signal corresponding to a total electrical current from said n transistors is output to a corresponding data line.
In accordance with still another aspect of the present invention, in a device or driver circuit as described above, each of said n transistors has a different size.
In accordance with still another aspect of the present invention, the emissive element referred to above is an organic electroluminescence element comprising an emissive layer containing an organic compound between an anode and a cathode.
In accordance with still another aspect of the present invention, the n transistors for outputting said current signal are formed on a substrate on which said organic electroluminecsence element and said thin film transistor are also formed.
According to the present invention, an active type emissive display device to which digital video data is input does not require a plurality of power sources respectively supplying a different analog gray scale voltage. Therefore, the number of input terminals and corresponding wiring patterns in a driver circuit can be reduced. When the driver circuit is integrated in the display panel, for example, the display panel can be downsized. Also, when the driver circuit is implemented as one chip (IC), the number of terminals of the IC can be decreased to thereby reduce the circuit scale.
Further, since a current signal in accordance with digital video data can be supplied, via a thin film transistor, to an emissive element such as an organic electroluminescence element to be driven by a supplied current, it is not necessary to convert a voltage signal in accordance with the digital video data into a current in a pixel portion, and thus effective drive of one emissive element by one thin film transistor can be implemented. The present invention is also applicable to a circuit configuration in which one emissive element is controlled by a plurality of thin film transistors within one pixel.